(1) Field of the Invention
The present invention relates to a novel extruded flexible magnetic strip for use in small DC motors. In particular, the present invention relates to an extruded flexible magnet which has a rectangular cross-section in the axial direction when curled. The present invention also relates to a novel extruder die for making the novel flexible magnetic strip.
(2) Prior Art
Extruded flexible magnetic strips are well-known in the art. Generally, these magnetic strips are made by blending in an extruder a magnetic compound, such as barium ferrite or a rare earth metal-cobalt, such as samarium cobalt, with an elastomeric composition, such as polyvinyl chloride, natural rubber, or polyurethane. To manufacture medium to low strength magnetic fields, magnetic isotropic particles are employed. To manufacture flexible magnet having strong magnetic fields, magnetic anisotropic particles are employed. While the present invention contemplates the use of either isotropic or anisotropic particles in making a flexible magnet, use of magnetic anisotropic particles is preferred.
If anisotropic particles are employed in the flexible magnets, it is necessary to properly orient the anisotropic particles in order to make a strong magnet. Orientation of the anisotropic particles may be achieved by mechanical and/or magnetic orientation, or a combination of these. The following U.S. Patent describes a prior art extrusion mechanical orientation process for making flexible magnetic strips having anisotropic particles therein.
U.S. Pat. No. 3,070,841 to Schornstheimer discloses an extruder for extruding flexible anisotropic magnets. The magnets are made from a composition having anisotropic particles bound by a plastic elastomeric material. By extruding the composition through the die, the anisotropic plate-like magnetic particles are oriented such that their magnetic axis is normal to the direction of extrusion. The efficiency of this orientation is further increased by subjecting the extrudate to further mechanical orientation by passing it between a pair of rollers. Therefore, the extrudate is subject to a dual mechanical orientation process. After the mechanical orientation, the extrudate is magnetized for end-use applications.
Conventionally, small brushless DC motors employ a magnet which has one or more pairs of opposite poles. The magnet is generally placed adjacent a shell which forms the rotor, although the shell and magnet could form the stator, depending upon the design of the DC motor. The magnet is typically formed in a strip by an extruder and rolled or curled into the shell, and anchored therein. Additionally, the magnet for a small DC brushless motor may also be manufactured in a tubular form so that it is inserted into the shell. This type of magnet has certain advantages over a strip magnet which is curled to fit within the shell in that tolerances for small DC motors are very small and a magnet initially formed in a tubular form can more easily be manufactured within the design tolerances. The following patent is exemplary of a tubular form magnet for small DC motors.
U.S. Pat. No. 4,327,346 to Tada et al. discloses an anisotropic polymeric magnet in tubular form in which the mggnetization orientation is directed in a direction perpendicular to the axial line of the tubular magnet. The tubular magnet is formed by injection molding which results in a very high degree of dimensional accuracy, compared to curling an extruded flexible magnet into the shell of a small DC motor.
Injection molded tubular magnets have the disadvantage of requiring different size molds for each size of magnet required in the plethora of small DC motors available in the marketplace. On the other hand, extruded strip flexible magnets can be curled into almost any desired diameter so long as the flexibility of the magnet will permit the curling to occur without cracking or breaking. Thus, while injection molded tubular magnets more precisely control the dimensions of the magnet, the process is not as economical or versatile as a strip extrusion process in which the magnetic material can be curled to fit within the shell.
Extrusion of flexible magnetic material causes several inherent problems, namely: (1) when extruding rectangular shapes from a rectangular die (as shown in FIG. 1), the material tends to be slightly thicker in the central portion of the elongated strip as opposed to the edge portions and is referred to as die swell (as illustrated in FIG. 2); and (2) the edge portions of the extruded material tend to have less oriented anisotropic particles as compared to the central portions. Therefore, the longitudinal edges have less magnetic strength than the central portion. These problems are further compounded when the elongated strip is cut and curled within a shell as illustrated in FIG. 3. The thicker central portion causes the edges of the flexible magnet to be out of contact with the shell resulting in poor circuit continuity. Additionally, the thicker central portion does not permit a uniform magnetic flux to exist within the air gap between the magnet and that portion of the DC motor which fits within the magnet (either the stator or rotor).
These well-known problems have been cured by: (1) designing the die exit opening such that the central area of the die is slightly smaller than the edges (as illustrated in FIG. 4), thus causing the extruded material to have a uniform thickness (as illustrated in FIG. 5); and (2) cutting off the edges of the extruded material to eliminate the portions containing a higher concentration of poorly oriented anisotropic particles, resulting in a flexible magnet having a more uniform magnetic field strength.
Even with the above solutions, using such an extruded elongated material in a small brushless DC motor causes other problems, namely, when the extruded flexible magnet is curled into a small arcuate or circular shape, as depicted in FIG. 6, the magnetic material tends to bend toward the center of the circle along its central elongated length causing the flexible magnet to: (1) only contact the shell along its two peripheral edges causing poor circuitry continuity; and (2) the central portion of the magnet bows inwardly causing a variable magnetic flux to exist between it and the stator.
Thus, there is a need to improve the extrusion process such that: (1) a curled extruded flexible magnet fully contacts the shell across the entire width and along the entire length of the extruded magnet yielding good continuity, and (2) produced a flexible magnet with a flat central portion, i.e., a uniform inside diameter along the entire peripheral circumference of the magnet so that more close tolerances between the rotor and the stator can be achieved yielding uniform magnetic flux in much the same fashion as is achieved by means of tubular formed magnets.
It is a chief aspect of the present invention to create an extrusion die which produces a flexible magnetic isotropic or anisotropic product, which upon being curled has both a flat exterior and interior circumference, and therefore solves the above noted problems.